adb: Don't clobber block devices during push

In the past, an adb push would ALWAYS unlink the target path.
Now, we only links and regular files (and links).
This allows the following to work:

   adb shell ls -l /dev/block/mmcblk0p8
     # brw------- root     root     179,   8 2013-11-28 07:43 mmcblk0p8
   adb push n7.jpa.mda.post_jb_encryption.raw  /dev/block/mmcblk0p8

Bug: 11984121
Change-Id: I982c7a1cd87986621f2bebdcb41c4e281f67f772